The sensitized photo-oxidation of n-pentadecane as a model for abiotic decomposition of aliphatic hydrocarbons in seawater

n-Pentadecane was photo-oxidized, as a surface film, on purified natural seawater under simulated environmental conditions with anthraquinone as sensitizer. Initial attack by hydrogen abstraction from carbon atoms within the chain is followed by reaction of the alkyl radicals thus generated with molecular oxygen. The stable reaction products identified by GC-MS strongly suggest that alkoxy radicals produced by cleavage of the peroxide bond are key intermediates in the decomposition of the original alkane. The alkoxy radicals stabilize themselves either as ketones by reaction with molecular oxygen which is transformed into HO₂ or, if μ-hydrogen atoms are available, by cyclic electron rearrangements similar to a Norrish type II photo-decomposition. The products of the electron rearrangements are 1-alkenes and daughter alkoxy radicals. The latter either yield methyl ketones and HO₂ radicals upon reaction with molecular oxygen or undergo further decomposition as long as γ-hydrogen atoms are available. The generation of acetone and HO₂ radicals, inferred by the proposed reaction mechanism as products of many such degradation sequences, offers a plausible explanation for their occurrence in seawater in addition to those already advanced.